There's no real conflict to this as of yet, just a moral dilemma I've been having.

I work in a fish market / sushi restaurant (I only work in the restaurant side myself these days). We have a large a la carte menu and then some combo meals that come with a bunch of different items, and you can generally switch any items of the same price tier. Whenever someone changes an item we ask if it's an allergy just to double check so we can prepare it safe if it is.

There is a pretty big Jewish neighborhood nearby and we do get a lot of locals. For whatever reason (I assume it's just easier, or maybe they think people will take an allergy more seriously than a restriction?) most guests tell us that they have an allergy to shellfish instead of telling us it's a preference or a restriction, including customers I am almost certain are Jewish. The problem with that is, if we're informed of a "shellfish allergy" we treat it as such, which doesn't necessarily mean that it's prepared separately from non-Kosher non-shellfish, and in some cases I have had said customers order non-shellfish items that aren't Kosher (or not switch those items out from a box). They technically didn't tell me they're eating Kosher though, so I can't figure out what my level of responsibility is here? I want to serve people food the way they want it prepared, but I also don't think "do you really have an allergy or are you Jewish?" is gonna go over that well in most cases.